1. Field of the Invention
The present invention is related to the field of electrical logging instruments used for evaluating wellbores drilled through earth formations. More specifically, the present invention is related to instruments used to determine the quantities and types of fluids flowing through wellbores.
2. Description of the Related Art
Wellbores drilled into petroleum reservoirs within earth formations for the purpose of extracting oil and gas typically produce the oil and gas from one or more discrete hydraulic zones traversed by the wellbore. When a wellbore is completed the zones are hydraulically connected to the wellbore. The oil and gas can then enter the wellbore, whereupon they can be transported to the earth's surface entirely by energy stored in the reservoir, or in combination with various methods of pumping.
Hydraulic zones penetrated by some wellbores can traverse a substantial length. In other wellbores a plurality of zones can be simultaneously hydraulically connected to the wellbore. In such cases, for the wellbore operator to maximize the efficiency with which the oil and gas are extracted from the reservoir it is useful to determine the rates at which oil, gas and other fluids such as water enter the wellbore from any particular point along the length of any particular hydraulic zone.
Various instruments are known in the art which can be used to determine the rates at which the fluids enter the wellbore from any particular point within any hydraulic zone. The instruments known in the art for determining the rates of fluid entry into the wellbore are called production logging tools.
Production logging tools are typically lowered into the wellbore at one end of an armored electrical cable. The tools can include sensors which are responsive to, among other things, the fractional volume of water filling the wellbore, the density of the fluid within the wellbore, and the flow velocity of the fluid in the wellbore. A record is typically made, with respect to depth within the wellbore, of the measurements made by the various sensors so that calculations can be made of the volumes of fluids entering the wellbore from any depth within the wellbore.
Methods known in the art for calculating the relative volumes of fluids entering the wellbore from production logging tool measurements generally require the use of laboratory determined models of the responses of the various production logging sensors to a range of volumetric flow rates of the different fluid phases in the wellbore. All of the sensor response models known in the art are based on an assumed "flow regime" of the fluids entering the wellbore. The flow regime is a descriptive name for the manner in which any or all of the individual phases of fluids in the wellbore travel along the wellbore, the phases typically being liquid oil, gas and water. A discussion of flow regimes can be found, for example in "A Comprehensive Mechanistic Model for Upward Two-Phase Flow in Wellbores", Ansari et al, Society of Petroleum Engineers, paper no. 20630.
A drawback to the methods known in the art for calculating the relative volumes of fluids entering the wellbore is that the methods known in the art do not account for the fact that the actual flow regime in the wellbore may be different from the particular flow regime assumed in the sensor response model. The calculations of relative volumes based on an assumed flow regime can therefore be erroneous.
It is known in the art to determine the flow regime by the use of iterative calculation techniques to fit the actual production logging tool measurements to a particular flow regime and then calculate the fluid volumes after determining the flow regime. Iterative calculation techniques can be difficult and time consuming to perform, and ultimately do not determine the flow regime to a high degree of certainty.
It is an object of the present invention to provide an apparatus and method for mapping the distribution of thermal properties of the fluids within the wellbore, so that the distribution of different types of fluids and consequently the flow regime in a wellbore can be determined to a high degree of certainty.